[threads] Use designated initializer syntax in MAKE_SPECIAL_STATIC_OFFSET

[threads] Use designated initializer syntax in MAKE_SPECIAL_STATIC_OFFSET

Originally we used a C90-style struct initializer `{ (index), (offset), (type)
}`.  Unfortunately, SpecialStaticOffset is defined with fields in a different
order on big-endian and little-endian machines.  Using a designated
initializer, we don't have to care about the order.